1. Field of the Invention
The present invention relates generally to a radar sensor which receives a radar wave, reflected from a target object and transmitted through a wave transmission member, in a plurality of antenna elements, and more particularly to the radar sensor which judges the existence of a wave attenuator attenuating the radar wave directed toward the member.
2. Description of Related Art
A millimeter wave radar sensor disposed on a vehicle has been used for a vehicle trailing control and/or a vehicle collision predicting control. In this radar sensor, a millimeter wave is transmitted from a transmitting antenna to a target object (e.g., an obstacle or a preceding vehicle) placed in front of the current vehicle, and the millimeter wave reflected from the object is received by a receiving antenna. Then, a distance between the current vehicle and the object and a relative speed of the current vehicle to the object are calculated from the transmitted wave and the reflected wave. Therefore, the current vehicle can automatically trail the preceding vehicle or predict a collision with the preceding vehicle with high precision.
To prevent the radar wave from being attenuated in a window of the vehicle during the transmission of the wave, the radar sensor is not located in the vehicle compartment but is disposed on the front bumper placed outside the compartment. In this case, the radar wave is not transmitted through the window, so that this sensor can prevent the attenuation of the radar wave caused by the window. However, the radar sensor is exposed to the open air, so that various things such as turbid water, snow, dust, mud and the like are easily attached to a surface of a protecting member covering the receiving antenna of the sensor. These things soil the surface of the protecting member so as to attenuate the reflected wave. For example, the soiling things absorb or reflect a large portion of the reflected wave so as to reduce the electric power of the wave received in the receiving antenna. Therefore, the performance for detecting the object in the radar sensor is undesirably degraded, so that the reliability in the trailing control and the predicting control is lowered in the radar sensor. When the sensor detects the degradation of the object detecting performance, various types of control requiring the detection results of the sensor are stopped or partially performed.
To detect the degradation of this performance caused by various things attached to the protecting member of the receiving antenna, Published Japanese Patent First Publication No. 2003-320866 discloses a radar sensor for estimating a degree of the degradation of the object detecting performance. In this sensor, a frequency spectrum of the received signal (i.e., reflected radar wave) is calculated, and the reception amplitude at the spectrum peak of the frequency spectrum is always monitored. When the surface of the protecting member is heavily soiled, the amplitude at the spectrum peak is lowered. Therefore, the degradation level of the performance can be estimated from a change in the amplitude of the spectrum peak. For example, when the change becomes larger than a threshold value, the radar sensor judges that the object detecting performance is degraded.
Further, various reflected waves (i.e., multi-path reflected waves) passing through different transmission paths are sometimes received in receiving antenna for a short time. In this case, the interference of the reflected waves is caused so as to temporarily reduce the amplitude at the spectrum peak. To distinguish the performance degradation caused by the soiled or dirty surface of the protecting member from the performance degradation caused by the multi-path reflected waves, the continuation time of the reduction in the amplitude of the spectrum peak is measured. When this continuation time exceeds a predetermined period, the radar sensor judges that the performance degradation is continuously caused by various things soiling the surface of the protecting member.
The radar sensor has a plurality of antenna elements each of which receives a reflected radar wave. Each antenna element corresponds to a receiving channel. This radar sensor can detect a target bearing (i.e., a direction to the target object). To make a small-sized radar sensor, the number of antenna elements has been recently reduced. However, as the number of antenna elements is decreased, the adverse influence of the performance degradation in one antenna element on the precision in the bearing detection is enlarged.
To maintain the precision in the bearing detection to a high level, it is required to reliably judge whether or not the surface of the protecting member is soiled or made dirty. For reliable judgment, it is desired to heighten the detection sensitivity to the surface of the protecting member soiled or made dirty. For example, even when the reduction of the amplitude at the spectrum peak is small, the sensor is adjusted to judge that the performance degradation is caused by the soiled or dirty surface of the protecting member.
However, when this detection sensitivity is heightened, the performance degradation actually caused by multi-path reflected waves is detected by mistake as the performance degradation caused by the soiled or dirty surface of the protecting member. As a result, even when the performance degradation of the radar sensor is not caused by the soiled or dirty surface of the protecting member, various types of control using the radar sensor are undesirably stopped in response to a misjudgment.